Field of Invention
The present invention relates to a reduction method of successive hard handoffs between base stations in CDMA mobile communication system, more particularly, to a reduction method of successive hard handoffs between base stations in order to prevent dropped calls or quality of communication from being deteriorated because of successive hard handoffs between base stations which frequently occur according to resources of the system.
Generally, there are handoffs of frequent occurrence between adjacent base stations in the CDMA mobile communication system and they are one kind of soft handoffs and hard handoffs according to the resources of the system. Since the successive handoffs have a bad effect upon the quality of communication, the dropped calls and a capacity of the system, it has been a point of issue to reduce the frequency of handoffs.
FIG. 1 is a diagram showing a multiple frequency situation of a general CDMA mobile communication system in which a terminating base station A(104) existing in a boundary region of the multiple frequency environment using the frequency of (Fh group+Fn group) in the dense subscriber region; a control station 1 (102) for controlling an adjacent base station B(105) whose frequency is in Fh group lower than those of the terminating base station A (104) and is in a non-dense subscriber region, a control station 2(103) for controlling the base station C(106); a switching center MSC(100) for exchanging communication data from the control station 1(102) or the control station 2(103); and a location register 101 for registering the location of each mobile station(107, 108) are included.
The handoff control procedure among mobile stations, base stations and control stations in the CDMA mobile communication system is done by the following three messages of a mobile station assisting handoff system.
First, a mobile station observes the pilot strength of a same frequency of the adjacent cell and transmits a pilot strength measurement message PSMM. The base station and the control station analyze the pilot strength measurement message PSMM, determine to construct another path(ADD procedure) or to restore one of previously established paths(DROP procedure) and then direct a handoff by transmitting an extended handoff direction message EHDM to the mobile station.
The mobile station receiving EHDM takes a proper action corresponding to the direction (for example, to indicate, to restore or to establish a new channel) and therefrom transmits a handoff completion message HCM. Under the condition of the PSMM message transmission of a mobile station starting a handoff in the control procedure, the adjacent base station has to have the same frequency bandwidth as the frequency of current call signals.
In the CDMA mobile communication system, the type of handoff is determined by the information concerning with the adjacent base station and the power strength measurement message received from the mobile station.
In the case of a mobile station being transferred to another sector of the same base station, a new channel resource is not allocated and a handoff is done by the channel of an existing call.
However, when a mobile station moves from the base station generating a current call to the direction of another base station, a channel of the new base station is allocated and a channel to the mobile station is established. In the case of soft handoff, a new channel is established under the condition of maintaining the channel having an existing call.
In the case of hard handoff, however, since all channels having the existing calls are recovered and at the same time a new channel is established the possibility of the deterioration of communication quality and the dropped calls will be high.
Therefore, except an inevitable case, the handoff of the code division multiple access (CDMA) method is the type of soft handoff if possible. When the pilot strength of an added base station is larger than that of the base station keeping the existing call and there is no channel corresponding to the corresponding frequency or frame offset in the added base station, the hard handoff is done.
A conventional reduction method of handoff is described in a reduction method of ongoing service handoff in a boundary region CDMA cell disclosed in Korea patent disclosure No. 96-27549(Jul.22, 1996) in which the handoff is done for the call in process in order to reduce the frequency of handoffs. The pilot strength and the distance to the mobile station are evaluated only for a new call. The reduction method is done by classifying the adjacent base stations into three cases of a omni base station, sector base station and combination of omni and sector base stations, and by blocking the call if the adjacent base station is identified as the region between the adjacent cells.
However, it is possible to block the call in order to reduce the frequency of handoffs but the possibility for the call of the mobile stations having a lot of boundary regions to be established is decreased. Therefore, it is proper to reduce the frequency of handoffs after establishing the call.
In a midtown, since the pilot strength between adjacent base stations is rapidly changed because of fading phenomena and the distance to the mobile station is not exactly evaluated, the method is not good enough to reduce the frequency of handoffs. Furthermore, there is a high possibility for the occurrence of hard handoffs between base stations as if playing ping pong.
On the other hand, as a conventional handoff reduction method, a method for achieving a minimum of a possibility that the mobile station in the boundary cell of the CDMA mobile communication station region moves out of the system boundary cell in ongoing service communicating is executed according to each of three cases of which the adjacent cell is an omni cell, a sector cell and a combination of omni and sector cells, and is explained in the followings.
In the case that the adjacent cell is an omni cell, when a call is received whether the received call is new or not is decided. If the call is not new (in the case of handoff), a general call processing procedure is provided. If the call is a new one, it is decided whether or not it is a boundary cell. If it is not a boundary cell, the distance between the mobile station and the cell antenna is evaluated. If the evaluated value is larger than a reference value, the call is blocked. If the evaluated value is not larger than the reference value, a general call processing service is provided.
In the case that the adjacent cell consists of sector cells, when a call is received whether or not the received call is a new one is decided. If the call is not new (in the case of handoff), a general call processing procedure is generally provided. If the call is a new one, it is decided whether or not it is either a boundary sector or to a boundary cell. If it is neither a boundary sector nor a boundary cell, a general call processing service is provided. If it is either a boundary sector or the boundary cell, the call is blocked.
In the case that the adjacent cells are the combination of omni cells and sector cells, when a call is received whether or not the received call is new is decided. If the call is not new (in the case of handoff), a general call processing procedure is provided. If the call is a new one, it is decided whether or not it is either a boundary cell or a boundary sector. If it is neither a boundary cell nor a boundary sector, a general call processing service is provided. If it is either a boundary cell or a boundary sector, the distance between the mobile station and the cell antenna is evaluated. If the evaluated distance is larger than a reference value, the call is blocked. If the evaluated value is not larger than the reference value, a general call processing service is provided. Therefrom, it reduces the possibility that the mobile station in the boundary cell can move out of the system boundary cell during ongoing service.
However, in the conventional handoff reduction method explained above, since the pilot strength received by the mobile station varies by a large amount because the balance of the transmitting power between base stations is not kept or a lot of high buildings stand close together are dense and it is difficult to evaluate the accurate distance to the mobile station, there may be the frequent handoffs among the adjacent base stations when a call is established. Furthermore, if there is no channel resource concerning to the corresponding frequencys or the frame offsets in the base station the hard handoffs may successively exist.
If the hard handoffs occurs in the same way as playing ping-pong between adjacent base stations when the adjacent base stations belong to the switching center different from each other, system load is increased. Furthermore, the temporary dropped call to a subscriber comes into existence because of frequently exchanging a switch according to the frequent hard handoffs between switching centers or the probability of call release according to message loss between switching centers is increased.